Key optical components of large aperture, space based optical instruments may be deployed on orbit to provide an aperture large enough to increase the resolution and optical performance by several orders of magnitude. The performance of such instruments depends on maintaining the precision and stability of the deployed structural geometry to within nanometers of an ideal shape. Nonlinear contact mechanics and freedom in the components of deployed structures mean that deployed instruments will have the capacity to change shape at the micron and nanometer level of resolution. Eliminating such nonlinearities as load path friction and freeplay would enable a deployed structure to be as linear and precise as a monolithic block of material.
In most mechanically deployed structures, components are moved from their stored positions into their final operational positions by some type of actuator and then locked into place with a deployment latch. For high precision structures, it is critical that the load paths and load predictable for the reliable operation of the instrument.
Existing deployable structure joints have several limitations that either completely prevent them from being used in high precision deployable instruments or require complex analysis and additional launch mass to provide deployment actuation and post deployment locking. Hinge joints previously used in moderate precision structures have relied on high levels of preload and friction to eliminate freeplay and geometric ambiguity. These joints have been shown to be unstable at the micron level, causing the structure to “micro-lurch” or change shape and thus move the instrument's optics far out of alignment.
Existing joints for precision space structures relied on high levels of preload between the many components to eliminate gaps and free play that cause inaccuracies in the structure. Unfortunately, these high levels of preload introduce correspondingly high levels of friction both during the deployment and after deployment has been completed. Friction mechanisms are nonlinear and thus are more difficult to control and less predictable.
Other hinge designs such as latch and actuator type systems suffer from the same disadvantages.
Recently, foldable truss members have been developed so that a truss structure can be collapsed and compactly packaged to save space during delivery and then released to expand and return to its original shape in orbit. All of these mechanisms add to the mass, expense and complexity of the structure and to the difficulty and expense of transporting it. These foldable members reduce the mass (and the delivery cost) of the structure by replacing the hinge, latch and actuator mechanisms with one single device. See, e.g., U.S. Pat. No. 4,334,391 incorporated herein by this reference.
Solid rods are joined on their ends forming a truss structure (a square frame for a solar panel array or a superstructure for a communications satellite antenna, for example) and pre-selected rods are cut in sections to form a hinge between the two sections. The rod sections are joined with spring steel elements similar to, if not actually, lengths of a carpenter's tape measure.
The rod sections can be folded with respect to each other by imparting a localized buckling force to one of the spring steel elements. Simply letting go of one rod section, returns the two rod sections to an end to end alignment due to the potential energy stored in the biased spring steel hinge elements.
In this way, a truss structure made up of several of these foldable rods can be designed on earth, collapsed for delivery to space, and then released once in position in space where the foldable rods flex back into position forming the truss structure designed and constructed on earth.
In use, this spring steel hinge design suffers from a number of shortcomings. First, hinges formed of spring steel elements require joining the ends of each spring steel element to a rod section. These joints and the spring steel elements themselves add significantly to the overall weight of the truss structure which is an undesired factor in space launch capability.
The spring steel elements also result in dimensionally unstable truss structures. The dimensional instability is caused by the relative motion of the internal components including the joints between the spring elements and the rod sections and permanent yielding of different areas of the spring elements themselves.
The result is that the shape of the truss structure may change when it is erected in space from the shape of the truss structure before it was collapsed on earth. This can have disastrous effects on instrument performance as even a ten nanometer to ten micrometer displacement can severely affect the performance of primary and secondary optics attached to the truss structure.
The inventors hereof have developed flexible material hinges or “strain-energy” hinges of various configurations which bend and fold and then, when released, automatically unfold. These types of hinges are used to fold and then deploy structures and structural system in which simplicity of operation and reliability of performance are greatly desired. Such systems have been used with great success in the deployment of spacecraft components such as solar arrays and antennae.
In traditional strain energy deployment, a flexible material such as a spring steel or thin fiber-reinforced plastic (composite) flexes to allow motion about a bending hinge line and is held in its stowed configuration until such time as deployment is desired. Once released, the stored strain energy in the flexed material provides the motive force to unfold the structure. In some systems, a lenticular or curved shape of flexing material is used so that once the flexing hinge straightens out, it locks into place with a curved shape that is significantly stronger in cross section than its bent, stowed shape.
Since the hinge is only flexing, there is no inherent friction to retard its deployment and, when such a folded hinge is released, the unfolding deployment action is very fast, almost violent.
Uncontrolled deployment of purely elastic systems results in extremely fast speeds at the end of deployment at latch-up that result in either excessive momentum loads being transferred into the structure, or an overload of the hinge as the momentum of the deployed article carries through to the latched hinge.
Existing solutions are either at the system level, providing mechanical restraint through a lanyard, or are at the material level, replacing the steel or composite hinge material with shape memory alloy or shape memory reinforced polymer. Both of these potential solutions have significant shortcomings that add to the overall weight and cost.
Mechanical restraint systems typically rely on a lanyard or linkage that connect the structure being deployed to an energy absorbing device such as an eddy current damper or visco-elastic damper. Devices such as these add to the mass and complexity of the overall system as well as complicating the kinematics of the deployment through the introduction of additional elements that must be managed and controlled.
Active materials such as shape memory alloys, shape memory plastics, and shape memory composites require the use of a heat source to raise their temperature above the phase change temperature. Once above the phase change temperature, the strain embedded in the material is released, causing actuation of the overall structure. However, the addition of the electrical components also adds to the cost and weight of the overall system and adds power, command and control requirements that previously did not exist. Additionally, active materials must make stiffness and density sacrifices in order to provide for the active material properties. The resulting mass and complexity additions reduce the advantages provided by the rate control inherent in the use of active materials.